An unmanned aerial vehicle (UAV) is an aircraft enabled to be flown remotely by a pilot, a navigator, or under computer control, typically without a human crew on board, and is often used for military applications. For the purposes of this application, a UAV is defined as an aircraft being capable of controlled, sustained, level flight and powered by a jet or reciprocating engine (e.g., propeller-driven). In some instances, the acronym UAV has been expanded to UAVS (Unmanned Aircraft Vehicle System), while the FAA (Federal Aviation Administration) has adopted the acronym UAS (Unmanned Aircraft System) to reflect the fact that these complex systems can include ground stations and other elements besides the actual air vehicles.
Regardless of the preferred acronym, UAV usage is growing exponentially, especially in the military, where the use and popularity of UAVs is growing at an unprecedented rate. For example, in 2005, tactical and theater-level unmanned aircraft (UA) alone had flown over 100,000 flight hours in support of Operation Enduring Freedom (OEF) and Operation Iraqi Freedom (OIF). Moreover, rapid advances in technology are enabling more functionality to be placed on smaller airframes, thus spurring a large increase in the number of Small Unmanned Aircraft Systems (SUAS) being deployed into the battlefield. However, due to the recent development and use of SUAS in combat, no formal Department of Defense reporting procedures have been established to track SUAS flight hours.
UAVs are available in an array of shapes, sizes, configurations, and characteristics; and, while historically UAVs were simple drones (remotely piloted aircraft), more recently autonomous control has been increasingly employed in modern UAVs. In fact, modern UAVs come in two basic varieties: (i) those that are controlled from a remote location; and (ii) those that fly autonomously by following preprogrammed flight plans, often using complex and dynamic automation systems.
As the capabilities grow for all types of UAVs, nations continue to subsidize their research and development, leading to further advancements enabling UAVs to perform a multitude of missions. As a result, UAVs are no longer limited to performing only intelligence, surveillance, and reconnaissance (ISR) missions, although these types of missions remain their primary use. Modern UAVs are often equipped to perform roles that include electronic attack (EA), strike missions, suppression and/or destruction of enemy air defense (SEAD/DEAD), network node or communications relay, combat search and rescue (CSAR), and various derivations of these themes. These UAVs range in cost from a few thousand to tens of millions of dollars, and the aircraft used in these systems range in size from a Micro Air Vehicle (MAV) weighing less than one pound to large aircraft weighing over 40,000 pounds.
UAV use is not limited to the armed forces; on the contrary, UAVs may be used in a growing number of civilian applications including, for example, firefighting and nonmilitary security work such as surveillance of pipelines and in disaster zones. Generally speaking, UAVs are frequently preferred for military and civilian tasks that are too dull, dirty, or dangerous for a manned aircraft. UAVs enabled to perform these tasks include, for instance, the Orion, Centaur, Skate, Excalibur, Rapid Eye, BAMS, Global Hawk, and Vulture systems—all designed by Aurora Flight Sciences Corporation, Manassas, Va., U.S.A., the assignee of the present application. For further information on the various Aurora UAV aircrafts and systems, see http://www.aurora.aero/.
When used for reconnaissance detail, a UAV can often loiter over an area for an extended period of time (e.g., a few hours) during a single mission, typically at low to medium altitudes, such as 2,000 to 3,000 ft. During these missions, major operational advantages are gained if the UAV is undetected by ground personnel. These advantages include, for example, the avoidance of an unwanted attack on the UAV, deterrence of behavioral changes of the target, and to maintain secrecy of reconnaissance UAV use.
To minimize the visual signature and prevent visual detection, UAVs may be small and/or camouflaged and, in many instances, operate in darkness. In addition, many UAVs are enabled to avoid detection by radar or infrared systems. However, despite these measures, a UAV can often be detected by its noise.
A first source of noise is the engine; however, the engine may be muffled using known technology. There are several ways to reduce the sound level from an aircraft engine. To soften engine noise, the flow of the turbulent exhaust gases can be smoothed out, or the exhaust pulsations can be modified. This may be accomplished by installing longer exhaust pipes where the last six to eight inches of the tail pipes are flattened and drilled with holes, thereby reducing sound emission. Although these tail pipe designs may help to alter the noise patterns, they are not true mufflers. A traditional aircraft muffler (e.g., those available from Gomolzig Flugzeug-und Maschinenbau GmbH, http://www.gomolzig.de/), may also be added to the exhaust system to significantly suppress or reduce engine combustion noises. In fact, a silencer, including, for example, a “Swiss-style muffler”, can be built that will reduce the noise to nearly zero. In Europe, due to the economic incentives to reduce noise (e.g., lower landing fees), it is rather common to equip smaller aircraft with these mufflers. Some mufflers may be installed under the cowl, and others may be in the airstream. For further information on Swiss-style mufflers, see, for example, the article entitled “Swiss style muffler”, available at http://www.piteraq.dk/flight/muffler.html. While mufflers, or silencers, are typically the preferred method, other solutions include reducing engine RPM and adding a turbocharger. Although these solutions typically result in power loss and weight increase, the net operational advantage of a quieter engine often makes the endeavor worthwhile.
Apart from engine noise, a second, and rather substantial, source of noise is created by the aircraft propellers which, unlike an engine, cannot be easily muffled. While some UAVs use jet propulsion, many UAVs are still propeller-driven and can therefore generate a significant amount of noise when a wake (typically formed by upstream flight surfaces) encounters a propeller, thereby increasing external noise levels. In fact, the noise emission of a UAV propeller (e.g., a pusher propeller) can be so strong that the propeller noise alone may be able to attract attention from ground personnel during a mission, thereby fomenting unwanted attention or a potential attack on the UAV.
Therefore, a need exists for a system and method for reducing the noise generated by a pusher propeller, enabling an aircraft to retain the advantages of its pusher propeller configuration, while yielding acoustic performance similar to that of the more traditional tractor propeller by reducing, or eliminating, wake formation and the resulting propeller-noise emissions.